You know what? So shoot, me down, but I don't think removing through traffic from London and shoving it underground is such a bad idea. The trouble with London (and the UK in general) is that we never get anything done properly. The aborted ringways project from the 70s (abandoned for the right reasons because it would have been done on the cheap and involved elevated motorways and massive destruction) has unfortunately left us with stubs of dual carriageways that attract cars like flies to dog turds, creating massive gridlock around the entrances to them. It would be better if they had never been built at all, but we're stuck with them now. Just think about what the roads round Hackney are like near the Hackney Wick interchange, or the Hell that is Earl's Court, Shepherds Bush & surrounds as scores of cars try to cram from all directions onto the stubs of the A12, A4 and A40 respectively. It's a nightmare both for air pollution, cyclists, pedestrians, and general sanity.
If those dual carriageways had never been built, then traffic would be leaving London in a much more spread out manner rather than clustering around honeypots, but dammit they were built, so we've got to deal with them as best we can.
So I say let's just finish the job, connect them all up, and free up the surface streets for cyclists, pedestrians, and local traffic.
Yes I KNOW the M25 filled up in 10 seconds, "building more roads generates more traffic", we need to invest in walking and cycling and public transport, and all that blah blah. But at the risk of sounding like a middle manager, this silo mentality from cycle campaigners that 'new roads always = bad for cyclists' is not always (though granted often) correct. If these tunnels were accompanied by a redistribution of roadspace in the rest of London towards cyclists - which at the moment is often dismissed because of lack of 'space' - then that would be a Good Thing, no?